This invention relates to inkjet printers and, in particular, to a printing technique for reducing direction-induced hue shift in a color inkjet printer.
Inkjet printers are well known and extremely popular. Details of a particular inkjet printer are described in U.S. Pat. No. 5,648,806, entitled Stable Substrate Structure for a Wide Swath Nozzle Array in a High Resolution Inkjet Printer, by Steven Steinfeld et al., assigned to the present assignee and incorporated herein by reference.
An inkjet printer ejects fine droplets of ink onto a print medium, typically paper, from precisely formed nozzles in one or more printheads. High quality color inkjet printers include printheads for the three subtractive primary color inks, cyan, magenta, and yellow, and a separate printhead for black ink. One type of color inkjet printer incorporates a separate replaceable print cartridge for each of the four colors of ink installed in a scanning carriage. Another type of color inkjet printer incorporates two, three, or four different color printheads in the same print cartridge. The order of printheads in the carriage is typically black on the left side or right side of the primary color cartridges, with the order of the primary color cartridges being arbitrary.
When printing a color image, droplets, or dots, of the three primary colors are printed in various combinations to achieve the desired color tones, or hues, to reproduce the original color image. Multiple dots of the same color may be used for a single color spot to increase the intensity of that color in the color spot. A color image to be printed is typically available in the memory of a computer connected to the printer, in a continuous color scale version of the color image, suitable for display on a computer monitor. The method of converting the continuous image to a representation in terms of discrete dots is commonly referred to as halftoning. Halftoning methods are described in the book Digital Halftoning by Robert Ulichney, the MIT Press, 1987, incorporated herein by reference.
In an inkjet printer, printing is performed as the printhead is scanned across a print medium. Since the printheads are generally much smaller than the image to be printed, the image is divided into regions of appropriate size to be printed in a pass of the scanning carriage across the print medium. The process of creating instructions to fire specific inkjet nozzles to print specific regions of the color image is often termed shingle masking or shingling. Shingling is also used to enhance print quality by laying down the ink dots in several passes.
In order to achieve high throughput, that is, to maximize the number of pages printed per minute, printing is performed bi-directionally. In bi-directional printing, each pass is printed in the direction opposite to that of the previous pass. Consider the case of single pass printing. A first region is printed as the carriage is scanned in a first direction across the print medium. Then the medium is advanced and the carriage is scanned across the medium in the opposite direction to print an adjacent region. Since the color printheads are in a fixed order in the carriage, when the carriage is scanned in the opposite direction, the component inks are printed on the paper in reverse order.
For example, if the pen order on the carriage is, from left to right, black, cyan, magenta, yellow, then when the carriage is scanned from left to right, yellow is printed first; then magenta is printed over yellow; cyan is printed over magenta; and finally black is printed over cyan. When the carriage is scanned from right to left, black is printed first, over which cyan, magenta, and yellow are printed respectively. This reversal of printing order between adjacent passes may result in a perceptible change in hue, or hue shift between passes. To produce a dark red color, for example, magenta and yellow dots are printed. As the carriage is scanned from left to right, magenta is printed over yellow. As the carriage is scanned from right to left, yellow is printed over magenta, which may produce a color with a yellowish cast, as compared to the color printed with magenta over yellow.
What is needed is a method for reducing the perceptible hue shift arising from bi-directional printing in a color inkjet printer.
Depleted shingle masks are substituted for the nominal ones, based on print direction, at the shingle masking stage, to compensate for print-direction-induced hue shift in a color inkjet printer. When direction-dependent depleted shingle masks are used, the number of ink drops of a given color printed per unit area depends on the direction in which the printhead is scanned across the print medium. According to an embodiment of the invention, a look-up table indexed by a composite color tone value formed from the original continuous color scale is used to provide a depletion probability. The depletion probability is defined as the probability of selecting a print-direction-dependent depleted shingle mask which, in turn, results in an appropriate overall level of depletion. In this embodiment, the depletion probability is compared with the error diffusion threshold value used in halftoning the continuous image to discrete dots to determine when the shingle mask pair associated with depleted ink volume is to be invoked. The method can be used with any halftoning process that includes an error diffusion threshold value or other random threshold generator.